A radio device from which communication is started transmits a confirmation signal to a counterpart, and if an acknowledgement signal is not returned from the counterpart, switches a communication mode to another communication mode. This operation is repeated until the acknowledgement signal is returned from the counterpart. Thus, the communication mode is matched with that of the counterpart, thereby establishing a communication link with the counterpart. A radio device caused to start communication by a second radio device receives a signal from the second radio device. The radio device switches a communication mode to another communication mode if synchronization is failed to be established. This operation is repeated until the synchronization is succeeded to be established. Thus, the communication mode is matched with that of the second radio device to be a communication counterpart, thereby establishing a link with the second radio device.